Update git url in installation instructions

Author: adamgraham

README.md

@@ -10,4 +10,4 @@ To load a package from a Git URL:
 2. Click the add (`+`) button in the status bar
 3. Select **Add package from git URL** from the add menu
 4. Enter the following Git URL in the text box and click Add:
-   `https://github.com/zigurous/unity-physics.git`
+   `https://github.com/zigurous/unity-physics-utils.git`